1. Field of the Invention
The present invention relates to a magnetoresistive element, a magnetic head,.a magnetic reproducing apparatus, and a magnetic memory, and more particularly to a magnetoresistive element having a structure adapted to flow a sense current in a direction perpendicular to the plane of a magnetoresistive film, and a magnetic head, a magnetic reproducing apparatus, and a magnetic memory using the magnetoresistive element.
2. Description of the Related Art
Recently, since a relative speed between a read head and a magnetic recording medium in reading information becomes lower with reduction in size and increase in capacity of a magnetic recording medium, it is expected to develop a magnetoresistive (MR) head capable of producing a high output even under a low relative speed.
On the other hand, it has been reported that a high magnetoresistive effect is realized in a multilayer film having a sandwich structure of ferromagnetic layer/nonmagnetic layer/ferromagnetic layer in the case where the ferromagnetic layers are not antiferromagnetically coupled. With regard to the two ferromagnetic layers sandwiching a nonmagnetic layer (referred to as a “spacer layer” or an “intermediate layer”), the magnetization of one ferromagnetic layer (referred to as “pinned layer” or “magnetization pinned layer”) is pinned by applying an exchange bias magnetic field, while the magnetization of the other ferromagnetic layer (referred to as “free layer” or “magnetization free layer”) can be reversed by an external magnetic field (for example, signal magnetic field). In this multilayer film, a high magnetoresistive effect can be obtained by changing a relative angle between the magnetization directions of the two ferromagnetic layers having the nonmagnetic layer interposed therebetween. The multilayer film of this type is referred to as a “spin-valve”.
The spin-valve can reach magnetization saturation in a low magnetic field, which is suited to an MR head, and has been already put in practical use. However, its magnetoresistive ratio is about 20% at maximum, and therefore a magnetoresistive element with a higher magnetoresistive ratio has been demanded.
The magnetoresistive element includes two types of structures, that is, a CIP (current-in-plane) type adapted to flow a sense current in a parallel direction to the film plane of the element, and a CPP (current-perpendicular-to-plane) type adapted to flow a sense current in a perpendicular direction to the film plane of the element. The CPP magnetoresistive element has been reported to show a magnetoresistive ratio of about 10 times that of the CIP magnetoresistive element, and it is not impossible to achieve the magnetoresistive ratio of 100%.
In the spin-valve structure, however, since the total thickness of spin-dependent layers is very small and the number of interfaces is few, the resistance itself is low and the absolute value of the output is also low when a current is supplied to the CPP magnetoresistive element in the perpendicular direction. In the case where a spin-valve having a structure used in the conventional CPP magnetoresistive element is supplied with a current in the perpendicular direction, the absolute value of the output, i.e., AΔR per μm2, is as low as about 0.5 mΩ μm2 for the pinned and free layers having a thickness of 5 nm. In other words, in order to realize a CPP magnetoresistive element using a spin-valve film, it is important to increase the output. To this end, it is very important to raise the resistance value of the portions related to spin-dependent conduction and increase the resistance change in the magnetoresistive element.
By contrast, to enhance the magnetoresistive (MR) effect, it has been proposed to insert a resistance adjusting layer containing an insulator in the spin-valve film (see J. Appl. Phys., 89, p. 6943, 2001, or IEEE Trans. Magn., 38, p. 2277, 2002). The spin-valve includes a portion related to spin-dependent scattering of electrons (pinned layer/spacer layer/free layer), and a portion with low spin-dependent scattering (buffer layer, antiferromagnetic layer, protective layer, etc.). When the resistance of the former portion is Rsd and that of the latter portion is Rsi, the magnetoresistive ratio MR of the spin-valve may be expressed as MR=.ΔRsd/(Rsi+Rsd). The aforementioned insertion of the resistance adjusting layer containing the insulator has aimed at the effect that the more Rsd is greater than Rsi, the higher is the MR.
The resistance adjusting layer includes a portion of an insulator where a current does not flow, and low-resistance portions (metal paths) through which the current flows. The current is confined toward the metal paths in the vicinity of the resistance adjusting layer. This is called a current confinement effect. It is only the vicinity of the resistance adjusting layer that the resistance is raised by the current confinement effect, and the spin-dependent scattering at a position remote from the resistance adjusting layer hardly contributes to the MR. Thus, it is effective to arrange a material exhibiting high spin-dependent scattering in the vicinity of the resistance adjusting layer.
However, since a material exhibiting high spin-dependent scattering is often poor in magnetic characteristics, the material is hard to use for a free layer (see Jpn. Pat. Appln. KOKAI Publication No. 2003-60263). In the spin-valve, excellent magnetic characteristics are required for the free layer. In order to read the signal magnetic field from the magnetic recording medium at high sensitivity, the free layer is required to have soft magnetic characteristics, such as a low coercivity, and low magnetostriction. These magnetic characteristics depend on the material and crystallinity of the free layer. To satisfy the magnetic characteristics in the material with a high MR, it is important to control the crystallinity of the free layer.
The following characteristics are required for the resistance adjusting layer inserted in the central portion of the spin-valve, according to the technique disclosed in J. Appl. Phys., 89, p. 6943, 2001 or IEEE Trans. Magn., 38, p. 2277, 2002. First, the insulating portion should be high-quality. In the CPP type magnetoresistive element, since a current is allowed to flow perpendicular to the film plane, the resistance adjusting layer is required to have high voltage endurance characteristics. Second, the conductive portions (metal paths) should also be high-quality. If the metal paths contain impurities by which electrons with a spin are scattered, the MR will be lowered. Thus, the metal paths should have the highest purity possible. The grain size and crystal orientation of a portion of the spin-valve located under the resistance adjusting layer determines the quality of the insulator and metal paths. Therefore, it is important to control the grain size and crystal orientation of the layer located under the resistance adjusting layer.
Accordingly, in the spin-valve including the insulator, it is important to control the grain size and crystal orientation in order to satisfy both the high MR and magnetic characteristics of the free layer.